Begin by melting the bittersweet chocolate chips or dark chocolate chips together with the evaporated milk in a medium saucepan over low heat. Stir continuously until the mixture is smooth and well combined. Remove from heat and let it cool slightly.
In a large mixing bowl, combine the crushed vanilla wafers, crispy rice cereal, and powdered sugar. Mix well to ensure that everything is evenly distributed.
Once the chocolate has cooled a bit, pour it into the bowl with the dry ingredients. Stir gently until everything is well incorporated.
Carefully fold in the miniature marshmallows, peanut butter chips, and chopped nuts of your choice. Be gentle to avoid breaking the marshmallows.
Line your 9x9-inch baking dish with parchment paper for easy removal. Spoon the fudge mixture into the dish, spreading it out evenly using a spatula.
Cover the dish with plastic wrap and refrigerate for at least 2 hours, or until the fudge is firm and set.
Once set, remove the fudge from the dish using the parchment paper. Cut into squares and serve. Enjoy your Dark Chocolate Marshmallow Nut Fudge Bars!
